One of the major methods of obtaining an intensity-modulated light signal is external modulation. Several types of external modulators can serve this purpose. Currently, the most highly developed external modulators are integrated electro-optic modulators using Lithium Niobate (LiNbO.sub.3) crystawl (e.g., a Mach-Zehnder modulator). With these modulators efficient light intensity modulation can be obtained by an applied voltage.
FIG. 1 shows a typical setup for measuring the optical response (i.e., the transfer curve) of such an electro-optic modulator as a function of the applied voltage. A CW laser light source (i.e., a carrier lighyt beam) with controlled polarization is coupled to a modulator using a polarization-maintaining (PM) optical fiber. As the applied bias voltage is swept, the optical response is measured using an optical power meter. A typical plot of the modulator transfer curve or function shows an optical response versus applied voltage characteristic which is sinusoidal and which has a short linear portion intermediate its ends (FIGS. 2A, 2B and 2C). For most linear analog applications, the input signal is imposed in this linear region. In order to achieve this, a DC bias voltage is applied to the modulator so that the DC optical response is about half-way between the minimum and the maximum ends of the curve. As shown by the left-hand portion of FIG. 2A, when an input signal (e.g., a high data rate signal, a data stream of bits, a radio frequency (RF) signal or an electric field signal derived from an electric field sensor) is applied to a modulator which is biased at the normal operating point or bias point, an output siganl with lowest harmonic distortion and intermodulation would result under ideal conditions.
Drift or variation in the bias point of LiNbO.sub.3 modulating devices is a serious problem which prevents stable operation. Three main types of bias drift are: bias variation due to temperature changes and the pyro-electric effect; slow drift caused by variations in the DC voltage applied to the set bias point; and optical damage due to the photo-refractive effect causing bias point changes. In order to maintain the modulator at its optimum operating point, these variations in the bias point must be compensated by varying the applied bias voltage.
FIGS. 2A, 2B, and 2C illustrate three different bias conditions. In FIG. 2A, the bias B.sub.1 is at the normal center point along the transfer curve. At this normal bias, a sinusoidal modulation signal superimposed on the bias voltage will result in a modulated optical output siganl with minimum distortion. FIG. 2B shows the condition in which the bias voltage B.sub.2 is displaced to a higher value, and the output waveform is severely distorted in the upper end. Similarly, FIG. 2C shows the condition in which the bias voltage B.sub.3 is displaced to a lower value and results in the optical siganl distortion at the lower end. It is desirable to operated the modulator at the normal bias point in order to maintain the fidelity of the signal.
As illustrated in FIG. 2B, when a pulsed RF signal is applied to the modulator having a bias point B.sub.2 in a nonlinear region or portion of the transfer curve, a distorted version of the pulsed RF light signal is obtained. Due to the clipping effect at the upper side, the output waveform is nonsymmetrical with respect to the DC line. This could be experimentally observed with a fast detector capable at the frequency of the RF carrier.
Several methods and control systems have been used to stabilize the bias of an optical modulator (e.g., see U.S. Pat. Nos. 3,780,296 to Waksberg et at.; 3,988,704 to Rice et al.; 4,071,751 to Waksberg; 4,306,142 to Watanabe; 4,253,734 to Komurasaki et al; and 4,977,565 to Shimosaka, and the paper by R. H. Buckley, "A rugged twenty kilometer fiber optic link for 2 to 18 gigahertz communications", SPIE EO/Fiber-90, San Jose, Calif.)
One relatively recent straight-forward method is to inject a continuous low-frequency pilot tone through the modulator (See U.S. Pat. No. 5,003,624 to Terbrock et al.) The amplitude of this tone is then detected by a photodiode at a fiber tap on the modulator output. The objective of this method and apparatus is to maintain the constant detected signal by changing the bias voltage accordingly. Due to its simplicity, this method has several drawbacks: the signal injected has to be large in order to be detected efficiently; the sensitivity to biasing drift is low at small signal levels; and the information on the slope of the transfer curve at the bias point is difficult to obtain.
Another methods uses a low frequency bipolar square wave electrically impressed on the modulator. (See M. G. Lee et al., "New robust bias control method for optical communications," SPIE Proceedings, Optical and Digital GaAs Technologies for Signal-Processing Applications, Vol. 1291, pages 55-65, April 1990.) At a fiber tap on the modulator output, a positive-going pulse and a negative-going pulse are detected. A control circuit then develops a bias voltage change in response to the amplitude differences between the two pulses. Although this method is more preferred than the previous one, the high amplitude of the low-frequency pulses can interfere with the signals sent by the modulator. The imposition of such a pilot tone results in an operation which inherently departs from the ideal bias point by the amount of the pulse amplitude. Therefore, the level of the pilot tone has to be kept at a low level to minimally disturb to the bias point.
Thus, there is need for a bias control means which can develop a bias voltage for an optical modulator and which overcomes the short-comings of the prior art.
SUMMARY OF THE INVENTION
A general object of the invention is to provide a bias control circuit for producing a bias signal for an optical modulator.
Another objective of the invention is to provide a circuit for converting the pulse-modulated RF optical output of an optical modulator into a bias signal which is a function of the symmetry of the RF pulses.
Still another object of the invention is to provide a system for modulating an incoming light beam in response to the operation of an optical detector which has a slow time response.
One specific object of the invention is to provide an apparatus and method for controlling the drift of the operating point in an optical modulator due to the pyro-electric effect and the photo-refractive effect.
In accordance with the present invention, claim is made of an electro-optical modulation system comprising: optical modulator means for pulse-modulating an incoming carrier light beamin response to a pulsed RF input signal and a bias signal and for producing a pulse-modulated optical output which is characterized by a plurality of enveloped pulses; optical detector means, using said pulse-modulated optical output from said optical modulator means, for producing an electrical signal which is representative of said output from said optical modulator means; signal producing means, using said electrical signal from said optical detector means, for producing pulse envelope signals which are representative of said envelopes of said pulses; and a bias control circuit, using said pulse envelope signals, for producing said bias signal as a function of said pulse envelopes.
The approach taken by the inventin in solving the problem of bias control in an electro-optical modulation system is simple and effective; it also offers the flexibility to maintain constant bias along any point on the transfer curve for other applications. For example, when the modulator is used as a frequency doubler, this can simply be done by maintaining the detected pulse at a certain amplitude and direction.
Numerous other advantages and features of the present invention will become readily apparent from te following detailed description of the invention, the embodiments described therein, from the claims, and from the accompanying drawings.